His taste for natural history developed early in an enthusiastic devotion to ornithology, and before he graduated he was sent by the Smithsonian Institution to collect birds in Labrador. Among his earliest writings are the account of this trip, and a treatise on the birds of the District of Columbia, both published in 1861, and both papers secured public recognition in England as well as in this country, thus making a beginning of his literary reputation. While yet a medical student, Dr. Coues was enlisted by Secretary Stanton as medical cadet, U. S. A., and served a year in one of the hospitals in Washington. On graduating in medicine in 1863, he was appointed by Surgeon-General Hammond for a year as acting assistant surgeon U. S. A. and, on coming of age passed a successful examination for the medical corps of the army. He received his commission in 1864, and was immediately ordered to duty in Arizona. His early years of service in that territory, and afterward in North and South Carolina, were utilized in investigating the natural history of those regions, respecting which he published various scientific papers. Though he wrote some professional articles, during his hospital experience, Dr. Coues seems never to have been much interested in the practice of medicine and surgery. After about ten years of ordinary military service as post surgeon in various places he was, in 1873, appointed naturalist of the U.S. northern boundary commission, which surveyed the line along the forty-ninth parallel from the Lake of the Woods to the Rocky mountains. In 1874 he returned to Washington to prepare the scientific report of his operations. He edited all the publications of the United States geological and geographical survey of the territories from 1876 to 1880 and contributed several volumes to the reports of the survey, notably his "Birds of the Northwest," "Fur Bearing Animals," "Birds of the Colorado Valley," and several installments of a universal Bibliography of Ornithology. The latter work attracted especial attention in Europe, and Dr. Coues was signally complimented by an invitation, signed by Darwin, Huxley, Flower, Newton, Sclater, and about forty other leading British scientists to take up his residence in London and identify himself with the British Museum. Dr. Coues also projected and had well under way a "History of North American Mammals," which was ordered to be printed by act of Congress when suddenly, at the very height of his scientific researches and literary labors, he was ordered by the war department to routine medical duty on the frontier. He obeyed the order and proceeded to Arizona, but found it, of course, impossible to resume a life he had long since outgrown. His indignant protests being of no avail, he returned to Washington and promptly tendered his resignation from the army in order to continue his scientific career unhampered by red tape.